Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2010 was about 22 million tons. About seventy percent of the ethylene oxide produced is further processed into ethylene glycol; about twenty percent of manufactured ethylene oxide is converted to other ethylene oxide derivatives and only a relatively small amount of ethylene oxide is used directly in applications such as vapor sterilization.
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, that is Ag-based epoxidation catalysts that contain small amounts of “promoting” elements such as rhenium and cesium.
Moderators, especially chloride moderators have long been used in the feed mixture for the gas phase production of ethylene oxide (see e.g., Law et al., U.S. Pat. No. 2,279,469, issued Apr. 14, 1942; U.K. Patent No. 1,055,147 issued Jan. 18, 1967, and Lauritzen, EPO Patent No. 0 352 850 B1, issued Jan. 19, 1994) and have been variously known also as “inhibitors”, “anti-catalysts”, and “promoters”.
While the moderator's role was not fully understood in these prior publications, it is well understood that the moderator plays a key role in maintaining the catalyst's selectivity—the efficiency of the partial oxidation of ethylene to ethylene oxide. This is especially the case with respect to rhenium-containing, high selectivity catalysts because for these catalysts the selectivity versus moderator concentration tends to be very steep and thus, small changes away from the optimal moderator concentration (this is the moderator concentration that provides the maximum measured or observed selectivity for a given temperature and catalyst in-service age) can produce significant decline in selectivity performance.
In addition to selectivity, the activity curve—as measured by the catalyst temperature necessary to maintain constant production—is also sensitive to moderator concentration. More specifically, the catalyst temperature is inversely proportional to moderator concentration, which means that lower catalyst temperatures can be obtained by continually increasing the moderator concentration. Despite this evident trade-off between selectivity and activity, those involved in designing, supplying or operating ethylene oxide plants have devoted significant attention and resources in an attempt to regulate moderator concentration so that maximum selectivity is achieved, even if it necessarily also means that the catalyst is operated at higher temperature than could be obtained with higher moderator concentrations.
The most straightforward way of finding the optimal moderator concentration (and hence the maximum selectivity) involves simple manual adjustments to the reactor feed and operator parameters—when the reactor temperature or feed composition is changed the operators adjust the moderator concentrations in small increments until the maximum selectivity is achieved. Any further increase in moderator concentration will cause the selectivity to decline. As an alternative to manual adjusting moderator levels, techniques for automated and computer-controlled regulation of the moderator levels have also been previously proposed in the prior art. For example, U.S. Pat. Nos. 7,657,331 and 7,657,332 recite specific formulas and ratios to predict what the optimal moderator levels should be, making use of a “Q value” for calculating the correct chloride concentration. This Q value is the ratio of the total “effective” moderator to the total “effective” hydrocarbon. The “effective” hydrocarbon value is determined by multiplying the molar concentration for each species of hydrocarbon by a correction factor that (according to theory) accounts for the differences in the ability of the different hydrocarbons to remove/strip reaction moderator (especially chlorides) from the surface of the catalyst; while the “effective” moderator value is determined by multiplying the molar concentration for each species of moderator by a correction factor that (again according to theory) accounts for the number of “active species” present in a specific moderator. These correction factors are determined for each individual moderator and hydrocarbon by what is, apparently, a complicated process of experimental trial and error. Indeed, the process for determining these correction factors is not set out with specificity in the aforementioned patents nor any actual examples of the procedure presented. A similar approach for automatically adjusting moderator levels as applied to more diverse moderator blends can be seen in U.S. Pat. No. 7,615,655. As mentioned previously, both procedures are extremely complex to implement and are unlikely to have broad applicability in actual plant operation.
Thus, despite the development of these and other techniques designed to maximize selectivity there is still considerable dissatisfaction from some plants operators with the performance and requirements of high selectivity catalysts. Particularly it has been noted that the performance of high selectivity catalysts is less stable than the prior generation of high activity catalysts and thus have an apparently shorter service life. An additional problem is that less steam is generated during the operation of high selectivity catalysts, and because ethylene oxide/ethylene glycol plants rely on the steam generated in the reactor in order to supply steam needed in other parts of the process, it may be necessary to import steam from OSBL, outside battery limits, to ensure proper plant operation. This problem can be even further exacerbated in areas with insufficient utility capacity. Accordingly, there is a continuing need for methods for operating high selectivity catalysts in an ethylene oxide plant wherein such methods promote improved stability performance and provide for more consistent production of steam in the reactor.